The Notch pathway constitutes a short-range communication channel involved in many fundamental aspects of multi-cellular life: proliferation, stem cells and stem cell niche maintenance, cell fate acquisition, differentiation and cell death. Mutations that cause misregulation or misexpression of Notch pathway proteins have been directly linked to multiple human disorders from developmental syndromes and cancer. In cancer, Notch is able to act as a tumor suppressor in tissues where it promotes differentiation or as an oncogene in tissues where it maintains precursors or promotes proliferation. Understanding the regulatory context that makes these opposing biological functions of Notch possible is a challenge;without this knowledge attempts to manipulate Notch signaling in order to gain a therapeutic advantage may help one tissue at the expense of several others. Given the important role of Notch in regulation of cellular differentiation and in stem cell maintenance Notch inhibition could potentially result in untoward side effects due to loss of Notch function in adult tissues in which Notch acts as a tumor suppressor. To better understand the tumor suppressor activity of Notch we propose an approach consistent with our long- standing interest in Notch signaling and skin development, which will utilize multiple genetic tools (all of which are currently available in my lab) to define the complex interactions facilitating multistage carcinogenesis in Notch-deficient skin. The first two aims are designed to define the TSG activity within the in vivo context and to provide mechanistic clues. The last two aims test mechanistic hypotheses.